The central premise of this proposal is that the lens has evolved several key protective and repair systems that maintain its transparent function and that an age-related decrease in the function of these systems contributes to age-related cataract. A hallmark of human lens aging and cataract formation is oxidation of protein methionines to protein methionine sulfoxide (PMSO). PMSO levels increase in the human lens with age and in human age-related cataract, 60% of total lens protein is found as PMSO. PMSO causes loss of protein function, protein aggregation and cell death. However, to date, the role of PMSO in lens aging and cataract formation has not been established. One key to unlocking the role of PMSO in lens aging and cataract formation is to identify those repair mechanisms that the lens has evolved to defend against PMSO damage and those proteins whose functions are lost upon methionine sulfoxide (MSO) formation. We have discovered that a novel PMSO repair enzyme called methionine sulfoxide reductase A (MsrA) is essential for lens defense against oxidative stress, viability, and defense against cataract formation. Since PMSO accumulates in the human lens with increasing age and since MsrA activity is essential for cataract resistance, it is likely that loss of MsrA repair of one or more lens proteins oxidized to PMSO upon aging and/or oxidative stress contributes to loss of protein function and cataract. A likely target for MsrA repair is ?-crystallin, which exists in the lens as an oligomer of two subunits (? A and ? B) each containing two conserved methionines. In addition to its role as a structural lens crystallin, ? -crystallin is essential for lens function through its ability to act as a molecular chaperone that protects lens proteins against age-related damage. This application will test the hypothesis that oxidation of ? -crystallin to methionine sulfoxide (? -crystallin-MSO) causes loss of chaperone function and that MsrA can repair and restore the chaperone function of ? -crystallin-MSO. Thus, loss of MsrA activity upon aging could result in increased levels of ? -crystallin-MSO, loss of lens chaperone function and ultimately cataract formation. The results of these studies will provide a novel mechanism for cataract development and an innovative model describing the interdependent functions of key lens protective and repair systems. The information gained from this work could have a major impact on the rational design of therapeutics based on increasing the activity of MsrA thereby preventing cataract formation.